This one really hits it and has some interesting analog vibe going on. The channels even have some crossover and interesting phasing (of a good nature).
It’s been a while since there has been an update to this, but that might be because there is not much to improve on. You may have noticed most of the plugins I own are from the “big-name / expensive” guys, but PSP is that rare indie exception.
Those graphs look pretty clean, a might too clean I would say. Not that I have a 1950s RS56 laying around, but I’m going to guess it was a bit more noisy and added a bit more distortion than this plugin.